Detection of bacteria is important in a variety of industries, including the food and beverage industry. For example, the need to screen food and water for pathogenic bacteria is crucial to ensuring consumer safety. The determination of levels of certain families of bacteria is a commonly used approach to estimating the shelf life and microbial acceptability of food products and hygienic status of the processing equipment and raw materials used in their manufacture. The diagnosis of microbial infections also relies on the detection of the causative organism(s).
There are many methods known for detecting bacteria. For example, bacteriophage, which are viruses that infect bacteria, may be employed. The presence of the bacteriophage, the infected bacteria, or the lack thereof, may be detected. Typically, a target bacteria is detected by infecting the bacteria with a bacteriophage (BP) specific to the bacteria, inactivating the excess BP, and then manipulating the BP-infected bacteria in some manner to detect the presence or absence of the BP as an indirect indication of whether or not the sample originally contained the target bacteria. Bacterial xe2x80x9chelper cellsxe2x80x9d can be used to amplify the number of BP-infected bacteria and thereby enhance, e.g., make more rapid, the assay method. A common detection method in the final stages of such an assay is to incubate the bacterial helper cells with the BP-infected bacteria and either observe changes in solution turbidity or, alternatively, observe BP plaque formation on an appropriate growth medium.
For example, U.S. patent application Ser. No. 09/434,586 (Wicks et al.) describes devices and methods for the detection of bacteria in a sample. Briefly, a sample containing suspect (target) bacteria is infected with a BP specific to the suspect bacteria, the excess BP is inactivated with an antiviral agent, and the BP-infected bacteria are added to bacterial helper cells to amplify the BP and to produce a signal that can be detected visually or with an instrument. For example, the BP can be detected by incubating the helper cells on agar and counting the number of BP plaques that are formed.
It would be very useful in such assay methods to employ enzyme substrates (ES) as indicators for detecting the presence of BP (and, thus, indirectly the presence or absence of target bacteria). Utilizing ES indicators could lead to significant advantages over trying to observe changes in solution turbidity or counting plaque formation. The use of ES indicators could lead to more convenient, more rapid, and less expensive assay methods. However, the use of traditional soluble ES indicators is generally not possible in such assay methods. The soluble ES would undesirably react with enzyme within the intact bacteria cells of both non-target bacteria and, if used, bacterial helper cells and thereby produce unacceptable levels of background signal.
The present invention solves the problem of the prior art by utilizing enzyme substrates as indicators that have been bonded (i.e., immobilized) to an insoluble solid support. The use of an immobilized enzyme substrate prevents the enzyme substrate from crossing a bacteria cell wall to react with enzyme within intact bacteria cells. As a result, the enzyme substrate can only react with an enzyme released from a lysed bacteria cell.
The present invention provides a method of detecting (identifying and/or quantifying) target bacteria. The method includes: combining bacteriophage and a sample of interest to form a reaction mixture; incubating the reaction mixture under conditions effective for the bacteriophage to lyse any target bacteria present in the sample of interest and release enzyme; adding an immobilized enzyme substrate to the reaction mixture; and monitoring the reaction mixture for a detectable signal produced from interaction between the immobilized enzyme substrate and any released enzyme present. Adding the immobilized enzyme substrate to the reaction mixture can occur before or after incubating the reaction mixture. This method can involve a qualitative or quantitative determination of bacteria in a sample.
In a preferred embodiment, the present invention provides a method of detecting target bacteria that involves: combining bacteriophage and a sample of interest to form a reaction mixture; allowing the bacteriophage to infect any target bacteria present in the sample of interest; adding an antiviral agent to inactivate any extracellular bacteriophage; adding bacterial helper cells to the reaction mixture; adding an immobilized enzyme substrate to the reaction mixture; incubating the reaction mixture under conditions effective for the bacteriophage to lyse any target bacteria present and the bacterial helper cells and release enzyme; and monitoring the reaction mixture for a detectable signal produced from interaction between the immobilized enzyme substrate and any released enzyme present. This method is preferably used for the quantitative determination of bacteria in a sample, although it can also involve a qualitative determination.
The present invention also provides an immobilized enzyme substrate that includes a porous solid support and an enzyme substrate covalently bonded thereto.